The invention relates to a method for detecting, in a combined digital signal, the presence of two signals differing in frequency, amplitude and phase and having frequencies occurring in a known series, such as in an R2 signalling system. Samples of the combined signal are multiplied by samples of at least one derivative of always one of the frequencies of the known series. These products are stored in accumulators until after a certain time the presence of certain frequencies is deduced from the sum of the products in the accumulators by means of comparison.
A method of this kind is known and can be used for various purposes. (see Australian published patent application No. 59138/73 published Feb. 10, 1975). One of the possible uses is for the analysis of digitally received MFC-signals (multi-frequency code signals). Another possible uses is in push-button voice frequency code signalling. Since the method can best be utilized in a digital connection, it is also imaginable to utilize the method in an analog connection, and because the apparatus utilizes digital techniques, it can be used in more than one way. Thus a digital result is immediately suited for further digital processing, if necessary together with other data, and a lower dissipation is often obtained. Although the analog-digital conversion may present difficulties in some cases, however in an analog connection one is free in choosing the parameters when sampling, such as the moments of sampling and the number of levels to be distinguished.